1. Field of the Invention
The present invention relates to an image recording apparatus, more particularly to an image recording apparatus that supplies adjustable driving current to a driven element by which an image is recorded, and to a method of recording an image using an apparatus of this type.
2. Description of the Related Art
Referring to FIG. 1, in an electrophotographic apparatus, a photosensitive member such as a photosensitive drum 51 is charged by a charging unit (CH) 35, then selectively illuminated by one or more light-emitting elements in, for example, a light-emitting-diode (LED) head 3 according to information to be printed, forming an electrostatic latent image on the photosensitive drum 51. The electrostatic latent image is developed by selective application of toner in a developer 52 to form a toner image, which is transferred to recording medium such as recording paper 53 by a transfer unit (T) 36, then fused to the paper. The elements shown in FIG. 1 form a type of printing mechanism 60.
One type of electrophotographic apparatus is an electrophotographic printer. A more detailed description of an electrophotographic printer will be given below with reference to FIG. 2, which is a block diagram of the control circuits of a conventional electrophotographic printer, and FIGS. 3 and 4, which are timing diagrams illustrating the operation of the conventional electrophotographic printer.
The printing control unit 1 in FIG. 2 is a computing device comprising a microprocessor, read-only memory (ROM), random-access memory (RAM), input-output ports, timers, and other facilities. Receiving signals SG1, SG2, etc. from a higher-order controller 55, the printing control unit 1 generates signals that control a sequence of operations for printing dot-mapped data given by signal SG2 (sometimes referred to as a video signal because it supplies the dot-mapped data one-dimensionally). The printing sequence starts when the printing control unit 1 receives a printing command from the higher-order controller by means of control signal SG1. First, a temperature (Temp.) sensor 43 is checked to determine whether the fuser 44 is at the necessary temperature for printing. If it is not, current is fed to a heater 44a to raise the temperature of the fuser 44.
When the fuser 44 is ready, the printing control unit 1 commands a motor driver 33 to drive a develop-transfer process motor (PM) 37, activates a charge signal SGC to turn on a charging power source 32, and thereby applies a voltage to the charging unit 35 to charge the surface of the photosensitive drum 51.
In addition, a paper sensor 41 is checked to confirm that paper is present in a cassette (not visible), and a size sensor 42 is checked to determine the size of the paper. If paper is present, a paper transport motor (PM) 38 is driven according to the size of the paper, first in one direction to transport the paper to a starting position sensed by a pick-up sensor 39, then in the opposite direction to transport the paper into the printing mechanism 60.
When the paper is in position for printing, the printing control unit 1 sends the higher-order controller 55 a timing signal SG3 (including a main scanning synchronization signal and a sub-scanning synchronization signal) as shown in FIG. 3. The higher-order controller 55 responds by sending the dot data for one page in the video signal SG2. The printing control unit 1 sends corresponding print data (HD-DATA) to the LED head 3 in synchronization with a clock signal (HD-CLK). The LED head 3 comprises a linear array of LEDs for printing respective dots (also referred to as picture elements or pixels).
After receiving data for one line of dots in the video signal SG2, the printing control unit 1 sends the LED head 3 a latch signal (HD-LOAD), causing the LED head 3 to store the print data (HD-DATA), then sends the LED head 3 a strobe signal (HD-STB), causing the LED head 3 to output light according to the stored print data (HD-DATA), thereby forming one line of dots in the electrostatic latent image. Output of the strobe signal (HD-STB) may overlap the transfer of the next line of the video signal SG2 and print data (HD-DATA), as illustrated in FIGS. 3 and 4.
Subsequent lines of print data are sent and received in the video signal SG2 in the same way. After each line has been stored, the LED head 3 is driven to emit light, selectively exposing the negatively charged surface of the photosensitive drum 51 to add another line of dots to the electrostatic latent image. When the printing control unit 1 activates control signal SG5, the developer power source 54 is switched on, applying a voltage to the developer 52, and negatively charged toner particles are attracted to the parts of the electrostatic latent image that were exposed to light, forming a toner image comprising black pixels (dots).
The photosensitive drum 51 continues to turn, carrying the toner image to the transfer unit 36. The high-voltage transfer power source 32 is turned on by control signal SG4 and supplies a positive voltage to the transfer unit 36, whereby the toner image is transferred onto the paper 53 as it passes between the photosensitive drum 51 and the transfer unit.
A temperature-humidity sensor 30 monitors the temperature and humidity inside the printer. The printing control unit 1 reads the temperature and humidity in the printer as necessary from the temperature-humidity sensor 30, thereby obtaining information about environmental conditions.
The printing control unit 1 has a table of transfer conditions corresponding to different ambient temperature and humidity conditions, and uses this table to select the optimum transfer conditions according to the environmental data read from the temperature-humidity sensor 30.
The paper 53 bearing the transferred toner image is transported to the fuser 44. When the paper 53 meets the fuser 44, the toner image is fused onto the paper 53 by heat generated by the heater 44a. Finally, the printed sheet of paper passes an exit sensor 40 and is ejected from the printer.
The printing control unit 1 controls the high-voltage transfer power source 32 according to the information detected by the size sensor 42 and pick-up sensor 39 so that voltage is applied to the transfer unit 36 only while paper 53 is passing between the transfer unit 36 and photosensitive drum 51. When the paper 53 passes the exit sensor 40, the printing control unit 1 turns off the high-voltage charging power source 31 and halts the developer-transfer process motor 37.
When a series of pages are printed, the above operations are repeated.
FIG. 5 shows the conventional circuit structure of an LED head 3. The print data signal HD-DATA and clock signal HD-CLK are received by a shift register 121 comprising, for example, two thousand four hundred ninety-six flip-flops FF1, FF2, . . . , FF2496 (this number of flip-flops is suitable for printing three hundred dots per inch on A4-size paper). The latch signal HD-LOAD is received by a latch unit 122 comprising a corresponding number of latches LT1, LT2, . . . , LT2496, which latch the data output by the shift-register flip-flops. The strobe signal HD-STB is supplied to a circuit 123 comprising an inverter G0, NAND gates G1, G2, . . . , G2496, and switching elements (transistors) TR1, TR2, . . . , TR2496 which are interconnected to drive a linear array of light-emitting elements (LEDs) LD1, LD2, . . . , LD2496 when the latch and strobe signals are both low, provided the print data output from the corresponding latches are high (indicating black dots or, more generally, high-intensity pixels). The transistors TR1, TR2, . . . , TR2496 operate as an array of driving elements, while the LEDs LD1, LD2, . . . , LD2496 operate as an array of driven elements. The power source of the current that drives the light-emitting elements is denoted VDD.
A problem encountered in this type of electrophotographic printer is that the size of a printed black dot depends on the number of other black dots nearby. Consequently, as the number of black dots in a given region varies, the blackness of the region (the total area occupied by the black dots) does not increase consistently. In particular, as the number of contiguous black dots in a rectangular region varies (this number is shown on the horizontal axis in FIG. 6), the area covered by the contiguous black dots (shown on the vertical axis) does not vary proportionally. As the number of black dots decreases, the size of the black area decreases more sharply (characteristic xe2x80x98axe2x80x99) instead of decreasing proportionally (characteristic xe2x80x98bxe2x80x99). The reason for this phenomenon has to do with the characteristics of the transfer unit.
A result of this problem is that when the printer prints very small letters, fine lines, and sparse (low-density) dither patterns, the printed lines and dots do not have the intended thickness (density) or size. Printing quality is degraded accordingly.
Similar problems are encountered in color electrophotographic printers: the size of a printed high-intensity (i.e., non-white) pixel is affected by the number of other high-intensity pixels in its vicinity. This phenomenon is also seen in image recording apparatus other than electrophotographic apparatus.
An object of the present invention is to form an image in which the total area occupied by dots in an image area is proportional to the number of dots in the area, regardless of the dot density in the area.
A further object of the invention is to form an image in which the total area occupied by dots in an image area is proportional to the number of dots in the area, regardless of ambient temperature and humidity variations.
A first aspect of the invention provides a method of recording an image by supplying driving current to driven elements according to pixel data indicating pixel intensity. The method includes the step of adjusting the driving current supplied to form a high-intensity pixel according to intensities of other pixels nearby. Preferably, the driving current is increased as increasing numbers of low-intensity pixels are disposed near the high-intensity pixel, and greater weight is given to pixels in close proximity to the high-intensity pixel than to pixels that are farther away. The method may also include the steps of sensing ambient temperature and/or humidity conditions, and adjusting the driving current according to these conditions.
The first aspect of the invention also provides an image recording apparatus that records an image by supplying driving current to driven elements according to pixel data indicating pixel intensity. The image recording apparatus includes an adjustment circuit that adjusts the driving current as described above, and may also include a sensor for sensing ambient temperature and/or humidity conditions, enabling the adjustment circuit to adjust the driving current according to those conditions as well.
More specifically, the first aspect of the invention provides an image recording apparatus having an array of driven elements, an array of driving elements that record an image by supplying current to the driven elements according to pixel data, and a compensation circuit that adjusts the driving current supplied by each driving element. The compensation circuit includes an adjustment circuit that adjusts the driving current supplied to form a high-intensity pixel according to intensities of other pixels nearby, preferably according to the intensities of pixels in an Mxc3x97N pixel block centered on the high-intensity pixel, where M and N are positive integers. The adjustment circuit may operate by comparing the Mxc3x97N pixel block with a plurality of prestored patterns having prestored compensated data values. Each prestored pattern may have a plurality of compensated data values, which are selected according to ambient temperature and/or humidity conditions.
The array of driven elements may comprise light-emitting elements for selectively illuminating a photosensitive member responsive to driving current, and the array of driving elements may be adopted to supply the driving current to respective light-emitting elements according to pixel data indicating pixel intensities, thereby forming an electrostatic latent image on the photosensitive member.
A second aspect of the invention provides a method of recording an image by supplying driving current to driven elements according to pixel data indicating pixel intensity, including the step of altering the pixel data by changing a low-intensity pixel to a high-intensity pixel, thereby enlarging a contiguous group of high-intensity pixels, if the low-intensity pixel is surrounded by a predetermined pattern of high-intensity and low-intensity pixels. Preferably, this method also changes a high-intensity pixel to a low-intensity pixel, thereby enlarging a contiguous group of low-intensity pixels, if the high-intensity pixel is surrounded by another predetermined pattern of high-intensity and low-intensity pixels. The altered pixel data may vary over a range of intensity levels, depending on the surrounding pattern of pixels. The intensity level may also be selected according to ambient temperature and/or humidity conditions.
The second aspect of the invention also provides an image recording apparatus that records an image by supplying driving current to driven elements according to pixel data indicating pixel intensity. The image recording apparatus includes an adjustment circuit that alters the pixel data by changing a low-intensity pixel to a high-intensity pixel as described above.
More specifically, the second aspect of the invention provides an image recording apparatus having an array of driven elements, an array of driving elements that record an image by supplying current to the driven elements according to pixel data indicating pixel intensities, and a compensation circuit that adjusts the driving current supplied by each driving element. The compensation circuit includes an adjustment circuit that alters the pixel data by changing a low-intensity pixel to a high-intensity pixel as described above, preferably by changing the central pixel in an Mxc3x97N pixel block according to the pattern of pixels in the block, preferably by comparing the Mxc3x97N pixel block with a prestored plurality of Mxc3x97N patterns, M and N being positive integers. The altered pixel data may vary over a range of intensity levels, depending on the surrounding pattern of pixels.
The array of driven elements may comprise light-emitting elements for selectively illuminating a photosensitive member responsive to driving current, and the array of driving elements may be adopted to supply the driving current to respective light-emitting elements according to pixel data indicating pixel intensities, thereby forming an electrostatic latent image on the photosensitive member.